MRI devices irradiate electromagnetic waves on a subject placed in a uniform static magnetic field generated by a magnet to excite nuclear spins in the subject and receive magnetic resonance signals as electromagnetic waves generated by the nuclear spins for imaging the subject. The irradiation of the electromagnetic waves and the reception of the magnetic resonance signals are attained by a device called RF antenna or RF coil, which transmits or receives radio frequency (RF) electromagnetic waves.
RF coils or antennas are roughly classified into two groups, those called surface antennas or local antennas and those called volume coils or volume antennas. Surface antennas have a round shape or planar shape, and show sensitivity in neighborhood of the antennas, and upon use, they are put on the surface of the subject. The volume antennas have a cylindrical shape or a two-disc shape consisting of two discs disposed up and down, and they show sensitivity in the whole space in the cylinder or between the discs, and upon use, a subject is placed in the space.
Among the volume antennas, those having a cylindrical shape are used in MRI devices called those of tunneled type, with which imaging is performed by entering a human lying on a bed into the inside of the magnet having a cylindrical shape. On the other hand, those in the shape of a pair of upper and lower discs are used in MRI devices called those of vertical magnetic field type, hamburger type or open type, with which imaging is performed by entering a human lying on a bed into the space between the upper and lower magnets.
Volume antennas having a cylindrical shape include those of a type called birdcage type (refer to, for example, Non-patent document 1) and those of a type called TEM type (refer to, for example, Patent documents 1 and 2). These volume antennas are usually provided with about 16 to 24 of rod-shaped conductors called rungs (crosspieces or crossbars of ladder) disposed along the side of the cylinder in parallel to the center axis of the cylinder. Capacitors are disposed in each of the rungs in order to resonate the antenna at the frequency of the electromagnetic wave irradiated on the subject. The number of capacitors to be disposed is usually at least 2×M, or about 6×M in the case of those having a larger number of capacitors, where M represents the number of rungs. For example, in the case of a birdcage type volume antenna having 24 of rungs, 24×6=144 of capacitors are used to constitute the antenna. As the number of capacitors increases, production cost and cost for quality control at the time of maintenance thereof increase.
As an attempt to reduce the number of capacitors aiming at reduction of the production and maintenance costs, there is an RF coil configured so that reception points are provided between a large ground plane and a ribbon-shaped conductor disposed with a distance from the ground plane (refer to, for example, Patent document 3). With such a configuration, a certain level of electric capacitance is effectually provided between the large ground plane and the ribbon-shaped conductor even if capacitors are not disposed between them as lumped elements, and therefore the number of capacitors can be reduced compared with an RF coil configured in a usual manner.
Although the technique disclosed in Patent document 3 is fundamentally directed to surface antennas, there is also described an example of volume antenna constituted with a large ground plane and a ribbon-shaped conductor in Patent document 3. In this example, the ring portions of the birdcage type antenna can be regarded as a large ground plane in the shape of divided two cylinders, and it is also regarded that rung portions are replaced with the ribbon-shaped conductor. Therefore, the fundamental configuration thereof is the same as that of a birdcage type antenna. However, in this volume antenna, the two cylindrical conductors carry out the same function as that of the ring portions of the birdcage type antenna, and therefore the cylindrical conductors do not serve as a perfect RF shield. As a result, the antenna shows sensitivity in the outside of the antenna having a cylindrical shape. Accordingly, when a conductor or a subject approaches from outside of the antenna, the sensitivity of the antenna significantly changes, and thus it is not suitable for practical use. Moreover, in this volume antenna, thickness of the dielectric material is a large factor for determining the resonance frequency, and therefore adjustment of the frequency is also difficult.
As an example of volume antenna having a reduced number of capacitors and not showing sensitivity in the outside of the antenna, there is a volume antenna having a large cylindrical ground plane, ring-shaped ground planes disposed at the positions of upper and lower openings of the cylinder and contacting with the cylindrical ground plane, and rings constituted with ribbon-shaped conductors disposed inside of the ring-shaped ground planes (refer to, for example, Non-patent document 2).